Determining oil-specific coefficients - MHT410 - MM70 - MMP8 - MMT162 - MMT310 - MMT330

Determining oil-specific coefficients for moisture in oil probes Technical Note
Document code
M212935EN
Revision
A
Language
English
Product
MHT410
MM70
MMP8
MMT162
MMT310
MMT330
Document type
Technical note

Follow this procedure to use your own apparatus for determining oil-specific coefficients for Vaisala moisture in oil probes, such as MMP8.

  • Apparatus for determining water content, for example, coulometric titrator and magnetic stirrer
  • Oil test station:
    • Temperature test chamber
    • For example, a conical flask (1 L) sealed by a PTFE stopper with an inlet for a moisture in oil probe
    • Moisture in oil probe, for example, MMP8
    • Magnetic stirrer

The equation of the ppm calculation is:

p p m w = a w × 10 ( B + A ÷ T )
where:
  • aw = Water activity
  • A, B = Coefficients
  • T = Temperature (K)

Follow this procedure to define the coefficients A and B for the equation:

L O G ( p p m s a t ) = B + A ÷ T
  1. Define the water content of the oil sample with the titration. Use the oil moisture level that is close to real conditions in the process.
  2. Measure the water activity of this sample with the probe at two temperatures that differ at least 20 °C. Follow the measurement stabilization of the probe from the interface it is connected to (for example, Vaisala Insight PC software).
    You must seal the sample very carefully. If the sample is in contact with ambient air, its water content will be affected.
    If the oil sample is very dry and the temperatures are close to each other, it may cause inaccuracy to the calculation model. To get the best possible performance, it is recommended to use oil conditions that represent real conditions in the application. Recommended values for the sample are approx. aw 0.5 at 20 °C.
  3. Define the correlation between aw, T and ppm (w/w) from the measured values. Calculate A and B according to the following example.
    A = L O G ( p p m s a t [ T 2 ] ) − L O G ( p p m s a t [ T 1 ] ) 1 ÷ ( T 2 ) − 1 ÷ ( T 1 )
    B = L O G ( p p m s a t [ T 1 ] ) − A ÷ T 1
    Table 1. Example: Measured water content 213 ppm
    T (K) aw ppmsaturation
    24.1 0.478 213/0.478 = 445.6067
    57.6 0.188 213/0.188 = 1132.979
    A = L O G ( 1132.98 ) − L O G ( 445.607 ) 1 ÷ ( 57.6 + 273.15 ) − 1 ÷ ( 24.1 + 273.15 ) = − 1189.4581
    B = L O G ( 445.607 ) − ( − 1189.4581 ) ÷ ( 24.1 + 273.15 ) = 6.6503583

Assumptions: The isoterm of water activity versus water concentration is linear and the solubility curve has the form of the given equation.